Orbital welding machines have become quite popular for welding pipelines, and especially for the welding of stainless steel pipelines as are commonly run in large plants, such as plants for manufacturing semiconductors. Stainless steel pipelines are used to carry a great variety of gaseous and liquid materials through a plant, and the construction of such pipelines requires the welding of a large number of fittings and long lengths of high purity stainless steel pipe. The quality of weld required in such applications mandates a reproducible weld of uniformly high quality, and orbital welding machines have been developed for just such applications.
Orbital welding machines rely upon an essentially constant flow of an inert purge gas flowing through the pipeline at a low, but constant pressure. However, in order to maintain a constant pressure of the gas, a very precise measuring technique is required. Commonly a low pressure gauge known as a Magnehelic gauge is used, and provides a visual indication of pressures ranging from about zero to about 10 inches of water. Most often, such gauges are connected to the pipeline being welded by means of a simple T-fitting temporarily inserted into the pipeline, with the stem of the fitting being connected by a piece of tubing or hose directly to the gauge. At one end the pipeline is connected to a source of inert gas such as a gas cylinder, while the other end of the pipeline has a restrictor to reduce the size of the open end and provide a constant back pressure to the flowing inert gas.
Experience has shown that while a good reading of the pressure may be obtained using a T-fitting, and a constant pressure can be maintained by closely watching the Magnehelic gauge, variations in the pressure in the pipeline nonetheless do occur, and cause problems with the quality of the weld. Moreover, the length of pipeline between the source of the gas and the restrictor varies from one weld to the next, and constant monitoring of the pressure is needed. Moreover, as a weld progresses around the circumference of the pipeline or fitting, the gap between the two ends being welded is gradually closed, and the escape of gas through the butted ends of pipe gradually diminishes until the weld is completed. Since there is inherently some loss at the butted ends, there is a gradual increase in the gas pressure in the pipeline until the weld is completed. Under some conditions, if the pressure is not properly controlled as the weld progresses, the pressure can rise sufficiently to blow through the weld.
Further, the greater the distance between the weld site and the T-fitting, the more likely is the weld to blow through.
The use of T-fittings performs satisfactorily in many applications, however a number of problems are also introduced by using such fittings for measuring the pressure of the inert gas. For example, the use of T-fittings also requires the removal and replacement of the T-fitting after a weld is completed and in preparation for the next weld. This can be not only time consuming, but allows for the introduction of impurities into the pipeline as the fitting is reused, or lays around ajob site until it is re-used. The purity of the pipeline and the weld and welding process are of major importance in many plants, in particular in semi-conductor plants where very small amounts of trace elements can cause unacceptable variations in quality control. Thus, each time a T-fitting is reused, an additional possibility of contamination arises.
Additionally, since the T-fitting is installed in the pipeline, it is generally installed downstream of the weld site at the location of the next weld, however the distance will usually vary between welds, such that the distance between the weld site and the T-fitting will no be constant from one weld to the next. As a result, more fluctuations in the purge gas pressure will occur requiring still further compensation in the gas pressure.
Accordingly, a primary object of the present invention is to provide a device for sampling the purge gas pressure at a weld site.
Another object of the invention is to provide a device for sampling the pressure of a purge gas flowing in a pipeline without introducing an additional fitting into the pipeline.
A further object of the invention is to provide a device for sampling the purge gas pressure flowing in a pipeline at the site being welded by an orbital welding machine.
Still another object of the invention is to provide a device for sampling the pressure of a purge gas flowing in a pipeline which may be quickly and easily attached to and removed from the pipeline.
Yet another object of the invention is to provide a device for sampling the pressure of a purge gas flowing in a pipeline being welded which minimizes the likelihood of introducing impurities into the pipeline.
These and other objects and advantages of this invention will become apparent from a detailed consideration of the following description and claims.